Pretty print YAML with Haskell.

ppy.hs

#!/usr/bin/env stack
-- stack --install-ghc --resolver nightly runghc --package bytestring --package yaml -- -Wall

import qualified Control.Exception as X
import qualified Data.ByteString.Char8 as B
import qualified Data.Yaml as Y
import qualified Data.Yaml.Pretty as Y
import qualified System.Environment as E
import qualified System.IO as IO

main :: IO ()
main = do
    files <- E.getArgs
    mapM_ safePrettyPrint files

safePrettyPrint :: FilePath -> IO ()
safePrettyPrint file = X.catch (prettyPrint file) (handleException file)

prettyPrint :: FilePath -> IO ()
prettyPrint file = do
    input <- Y.decodeFile file
    let output = Y.encodePretty config (input :: Maybe Y.Object)
    B.writeFile file output

config :: Y.Config
config = Y.setConfCompare compare Y.defConfig

handleException :: FilePath -> Y.ParseException -> IO ()
handleException file exception = do
    let message = concat
            [ file
            , ": "
            , Y.prettyPrintParseException exception
            ]
    IO.hPutStrLn IO.stderr message